Phosphorescent blue organic light-emitting diodes with new bipolar host materials

Article abstract of DOI:10.1166/jnn.2012.4586

We report narrow band gap bipolar host materials, CbPr-3 (9,9'-[(3,3'-Biphenyl-3.3'-yl-bipyridine)- 1,3-biphenyl]bis-9H-carbazole) and Bim-4 (9,9'-[5-(1-phenyl-1H-benzimadazol-2yl)-1,3-phenylene] bis-9H-carbazole), for blue phosphorescent OLEDs applicatio

Full text of DOI:10.1166/jnn.2012.4586

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Journal of
Nanoscience and Nanotechnology
Vol. 12, 1361–1364, 2012
Phosphorescent Blue Organic Light-Emitting
Diodes with New Bipolar Host Materials
Eun-Sun Yu1, Sung-Hyun Jung1, Mi-Young Chae1, Woo Sik Jeon2,
Chandramouli Kulshreshtha2, Jung-Soo Park2, and Jang Hyuk Kwon2ꢀ∗
1Cheil Industries Inc., Gocheon-Dong, Uiwang-Si, Gyeonggi-Do 332-2, Republic of Korea
2Department of Information Display, Kyung Hee University, Dongdaemoon-Gu, Seoul 130-701, Republic of Korea
We report narrow band gap bipolar host materials, CbPr-3 (9,9ꢀ-[(3,3ꢀ-Biphenyl-3.3ꢀ-yl-bipyridine)-
1,3-biphenyl]bis-9H-carbazole) and Bim-4 (9,9ꢀ-[5-(1-phenyl-1H-benzimadazol-2yl)-1,3-phenylene]
bis-9H-carbazole), for blue phosphorescent OLEDs application. These two bipolar hosts have high
triplet energy of ≥ 2ꢀ9 eV, capable of reducing the driving voltages and improving efficiencies. Sig-
nificant low driving voltages of 7.4 and 6.6 V were obtained for CbPr-3 and Bim-4 hosts, compared
with 9.0 V of the commonly used host, mCP (1,3-bis(9-carbazolyl)benzene). At a given constant
luminance of 1000 cd/m2, the power efficiency of both the bipolar host devices was enhanced by
2.5 times.
Keywords: Phosphorescent OLED, Bipolar Host, Triplet Energy.
1. INTRODUCTION
red PHOLEDs but the blue phosphorescent bipolar host is
still a challenge that can lead to excellent performance of
the devices.
Organic light-emitting diodes (OLEDs) in recent decades
have proven their potential with superior properties for low
power consumption displays, lightings and printed elec-
tronics industry.1ꢀ2 OLED lifetime has been dramatically
improved over the last ten years with excellent progress
in OLED materials and device technologies. On the other
hand, power efficiency improvement in blue OLEDs is not
so rapid. In order to improve power efficiency, high inter-
nal quantum efficiency with phosphorescent emitter and
more importantly techniques to gives low driving voltage
in the devices are required. In the past, several approaches
were employed to enhance power efficiency of phospho-
rescent OLEDs (PHOLEDs), which include doping or
designing charge transporting layer,3ꢀ4 mixed host emissive
layer,5 high triplet energy host material6 etc. Generally,
unbalance hole and electron currents in the devices makes
the recombination zone narrower, thereby shifting it either
close to hole transport layer (HTL) or electron transport
layer (ETL). Thus narrower charge recombination zone in
PHOLEDs are prone to triplet–triplet exciton quenching
due to local high density of the triplet excitons and also
long diffusion of the triplet excitons.7ꢀ8 Therefore, bipolar
host molecules capable of transporting both charges can
meet the requirement of balancing the charge density for
simplification of high performance PHOLEDs. Although
several bipolar host materials are available for green and
Over the past few decades, a very few blue host mate-
rials based on pyridine or phosphine oxide derivatives
have been reported with the ideal EQE value of ∼20%.
Recently, highly efficient phosphine oxide derivatives,
SPPO1,9 BCPO10 for sky blue, and PPO2111 for deep
blue PHOLED application have been demonstrated suc-
cessfully. The devices based on these materials have shown
the EQE of about 20.5%, 23.5%, and 19.2%, respectively.
Other pyridine containing bipolar host, 26DCzPPy12 has
been reported with 25% EQE for sky blue PHOLED appli-
cations. Although these materials were used to obtain high
quantum efficiencies, but, there are some deficiencies that
hampered their further applicability, such as operational
stability, color chromaticity, thermal and morphological
instability etc.
Herein, we report the new bipolar host materials, CbPr-3
(9,9ꢀ-[(3,3ꢀ-Biphenyl-3,3ꢀ-yl-bipyridine)-1,3-biphenyl]bis-
9H-carbazole) and Bim-4 (9,9ꢀ-[5-(1-phenyl-1H-benzi-
madazol-2yl)-1,3-phenylene]bis-9H-carbazole) for blue
PHOLED applications. In CbPr-3 and Bim-4, pyridine
moieties and imidazole moieties were inserted to enhance
electron transporting capability, whereas carbazole moi-
eties functions as hole transport units. Both these materials
possess high triplet energies and narrow band gap result-
ing in good device efficiency and low driving voltage
characteristics in blue PHOLEDs.
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 2
1533-4880/2012/12/1361/004
doi:10.1166/jnn.2012.4586
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Phosphorescent Blue OLEDs with New Bipolar Host Materials
Yu et al.
2. EXPERIMENTAL DETAILS
in the 20 ml of tetrahydrofuran and 20 ml of toluene,
then added with a solution of 20 wt% of tetraethyl ammo-
nium hydroxide dissolved in 20 ml of water. The mixture
obtained was heated and refluxed for 9 hours. The reac-
tion fluid was separated into the two layers, and an organic
layer thereof was cleaned with a sodium chloride saturated
aqueous solution and later dried with anhydrous sodium
sulfate. Subsequently, the organic solvent was removed by
distillation under reduced pressure The residue was puri-
fied by silica gel column chromatography to provide 4.5 g
of the intermediate (C) (yield 70%).
2.1. Material Synthesis
The compound, CbPr-3 was synthesized according to the
procedure shown in Figure 1. The synthesis of Bim-4
was done as described previously.13 Molecular structure of
Bim-4 can be seen inset of Figure 2.
Step 1 Synthesis of intermediate (A): Carbazole 5.1 g,
1,3,5-tribromobenzene 19.1 g, cuprous chloride 0.8 g and
potassium carbonate 16 g was suspended in 40 ml of
dimethy sulfoxide(DMSO), and heated and refluxed for
8 hours. The reaction fluid was cooled to room temper-
ature and re-crystallized with methanol. The precipitated
crystals were separated by water filtration and the obtained
residue was purified by silica gel column chromatography,
providing 7.3 g of the intermediate (A) (yield 50%).
Step 2 Synthesis of intermediate (B): 7.3 g of the inter-
mediate (A) was dissolved in 40 ml of tetrahydrofuran,
then 10 ml (1.6 M) of n-butyl lithium hexane solution
Step 4 Synthesis of CbPr-3: 4.5 g of the intermediate
(C), 2.0 g of 3-pylidineboronic acids, and 0.2 g of tetrakis-
(triphenyl phosphine) palladium were suspended in 20 ml
of tetrahydrofuran and 20 ml of toluene, then added with a
solution of 20 wt% of tetraethyl ammonium hydroxide dis-
solved in 20 ml of water. The obtained mixture was heated
and refluxed for 9 hours. The reaction fluid was separated
into two layers, and an organic layer thereof was cleaned
with a sodium chloride saturated aqueous solution and
dried with anhydrous sodium sulfate. Subsequently, the
organic solvent was removed by distillation under reduced
pressure, and the residue was purified with silica gel col-
umn chromatography to provide 2.9 g of the CbPr-3 (yield
60%). Mass spectroscopy analysis data of CbPr-3 is as fol-
lows: MS (ESI) m/z 639.18(M+H)+ Calc. for C46H30N4:
ꢁ
(1.6 M) was added at −70 C The obtained solution was
agitated at −70 ꢁC to 40 ꢁC for one hour. And 5 ml of iso-
propyl tetramethyl dioxaborolane was slowly added. The
obtained solution was heated to room temperature, and
then agitated for 6 hours. To the obtained reaction solution,
20 ml of water was added and agitated for 20 minutes.
The reaction solution was separated into two liquid lay-
ers, and organic layer thereof was dried with anhydrous
sodium sulfate. After removing the organic solvent under
a reduced pressure, the obtained residue was purified with
the silica gel column chromatography to provide 5.4 g of
the intermediate (B) (yield 67%).
1
638.78, H NMR (300 MHz, CDCl3ꢁ ꢂ (ppm) 8.9(d, 2H),
8.65 (d, 2H), 8.18 (d, 4H), 8.21–7.88 (m, 7H), 7.81 (s,
1H), 7.6 (m, 4H), 7.50–7.48 (m, 4H), 7.42–7.32 (m, 2H),
7.31–7.29 (m, 4H); 13C NMR (75 MHz, CDCl3ꢁ 149.29,
148.41, 143.99, 141.22, 140.55, 140.09, 139.96, 135.84,
134.59, 126.29, 126.10, 125.84, 124.60, 123.70, 120.57,
120.52, 109.64.
Step 3 Synthesis of intermediate (C): 5.4 g of the inter-
mediate (B), 3.3 g of 1,3,5-tribromobenzene, and 0.3 g of
tetrakis-(triphenyl phosphine) palladium were suspended
Thermal properties of CbPr-3 and Bim-4 were investi-
gated by thermogravimetric analysis (TGA) and differential
Fig. 1. Synthetic method of CbPr-3.
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Yu et al.
Phosphorescent Blue OLEDs with New Bipolar Host Materials
Table I. A summary of energy levels.
scanning calorimetry (DSC). Both these compounds
exhibit high thermal-decomposition temperatures (Tdꢁ of
mCP
CbPr-3
Bim-4
ꢁ
about 350 and 511 C. Their glass transition temperatures
HOMO (ev)
LUMO (ev)
Band gap (ev)
Triplet energy (ev)
5.9
2.4
3.5
2.8
5.8
2.6
3.2
2.9
5.8
2.4
3.4
3.0
ꢁ
(Tgꢁ are about 120 and 139 C which are higher than that
of mCP.14 The high value of Td and Tg proves excellent
thermal and morphological stability, enabling the prepara-
tion of homogenous and stable thin film.
cyclic voltammogram. The LUMO energy levels of both
the hosts are at 2.6 and 2.4 eV which can be deduced
from the calculation of band gaps. Table I summarizes the
experimentally obtained energy levels.
2.2. OLED Fabrication and Measurement
1,4-Bis(1-naphthylphenylamino)-biphenyl (NPB) and
mCP as the hole transporting material or host were com-
mercially available. Other bipolar hosts, CbPr-3 or Bim-4,
were synthesized in the laboratory. The phosphores-
A series of the devices were fabricated and evalu-
ated for their J–V and L–V output characteristics. The
devices were fabricated using CbPr-3, Bim-4, respec-
tively. For comparison, mCP type of host device with
10 wt% of FIrpic in similar experimental conditions was
also fabricated. These devices were fabricated on the ITO
with a multilayered structure of ITO/NPB (40 nm)/mCP
(15 nm)/Host: 10 wt% Firpic dopant (30 nm)/TPBI
(25 nm)/LiF (1 nm)/Al (100 nm). Here NPB improves the
hole injection. The operating condition and characteristics
of these devices are compared and summarized in Table II.
The mCP, CbPr-3, and Bim-4 host devices are named as
device A, B, and C.
Figure 3 shows the current density–voltage (J–V ) and
luminance–voltage (L–V ) characteristics of all the three
devices. The devices B and C have lower driving voltages
compared with the reference device A. It was observed
that driving voltages to reach 1000 cd/m2 were found to
be 7.4 and 6.6 V for the devices B and C, lower than
9.0 V for mCP based device. The reduction in driving
voltage in device B and C is attributed to minimization
of charge trapping by dopant molecules in a narrow band
gap host emitting layer and lower injection barrier to the
emitting layer. In case of mCP as a host material, its hole
transportation is faster than that of electron, and its wide
band gap introduces charge trapping by dopant molecules
at deep sites. Therefore, there is unbalancing of charges
which make the recombination zone narrower, thereby
shifting more towards electron transporting layer. This can
cent dopant material iridium(III)bis(4,6-(difluorophenyl)
ꢀ
pyridinato-N,C2 picolate) (Firpic), and electron trans-
porting material, 1,3,5-tris(N-phenylbenzimidazol-2-yl)
benzene (TPBI) were commercially available. The details
of fabrication and measurement has been described
elsewhere.15
3. RESULTS AND DISCUSSION
The photophysical properties of both the host materials
were measured using UV-visible absorption and photolu-
minescence spectra as shown in Figure 2.
The absorption spectrum of CbPr-3 and Bim-4 have
absorption bands from 230 to 370 nm which could be
from ꢃ–ꢃ∗ transitions of the conjugated ꢃ-electron sys-
tem. The PL emission maxima for CbPr-3 and Bim-4 in a
solution were observed around 391 and 400 nm. The low
temperature phosphorescent spectrum of the both materials
was measured in 2-methyl tetrahydrofuran and methylene
chloride mixed solution at 77 K, revealed the phosphores-
cent sub-band of two peaks. From the first peak, triplet
energies of 2.9 and 3.0 eV were calculated. It confirms
that CbPr-3 and Bim-4 triplet energies are in a good agree-
ment in transferring excitons from the bipolar hosts to the
Firpic dopant having triplet energy of 2.65 eV.12 Cyclic
voltammetry experiments were performed to know HOMO
levels of CbPr-3 and Bim-4. The HOMO energy levels
for CbPr-3 and Bim-4 were estimated to be 5.8 eV by
Table II. A summary of device characteristics.
Device A
(mCP)
Device B
(cbpr-3)
Device C
(Bim-4)
Turn-on voltage
(at 1 cd/m2)
Operating voltage
(1000 cd/m2)
Efficiency
4.4 V
9.0 V
3.4 V
7.4 V
3.0 V
6.6 V
11.0 cd/A
3.8 lm/W
15.0 cd/A
7.8 lm/W
9.6%
17.5 cd/A
9.4 lm/W
20.3 cd/A
11.5 lm/W
13.0%
20.3 cd/A
9.7 lm/W
22.0 cd/A
15.0 lm/W
14.1%
(1000 cd/m2)
Efficiency
(maximum)
External quantum
efficiency (%)
CIE(xꢀy)
(1000 cd/m2)
(0.17, 0.28)
(0.17, 0.29)
(0.17, 0.29)
Fig. 2. Absorption and photoluminescence spectra of CbPr-3 and
Bim-4. Inset shows the molecular structure of Bim-4.
J. Nanosci. Nanotechnol. 12, 1361–1364, 2012
1363

Phosphorescent Blue OLEDs with New Bipolar Host Materials
Yu et al.
power efficiency at a high current density. In a reference
device, we have poor charge balance leading to low effi-
ciency and steep efficiency roll-off. It clearly reveals that
external quantum efficiency (EQE) for CbPr-3 and Bim-4
devices were 13.0% and 14.1% higher than mCP based
device exhibiting EQE of about 9.6%.
4. CONCLUSION
In summary, we have demonstrated superior performance
of CbPr-3 and Bim-4 as the bipolar narrow band gap hosts
for blue PHOLED application. The minimized charge trap-
ping by a dopant molecule and a low injection barrier to
emitting layer were resulting into significant reductions
in driving voltage and improvement of efficiencies. For
CbPr-3 and Bim-4 hosts, the driving voltage was reduced
about 1.6 and 2.4 V while their power efficiency at con-
stant luminance of 1000 cd/m2 was enhanced 2.5 times
with low roll-off.
Fig. 3. J–V and L–V characteristics of the devices.
Acknowledgment: This work was supported by a grant
from the Human Resources Development of the Korea
Institute of Energy Technology Evaluation and Planning
(KETEP) funded by the Korea government Ministry of
Knowledge Economy.
Fig. 4. Current and Power efficiencies of the devices.
enhance the possibility of triplet–triplet annihilation also.
Thus the mCP host device exhibits poor performance as
well as shows high driving voltage. Inset of Figure 3 also
shows the EL spectra of all the three devices exhibit-
ing emissions in the blue regions. It indicates complete
energy transfer from the host materials to the guest due
to confinement of triplet excitons at the Firpic dopant
molecule. The EL spectral peaks and CIExy coordinates of
all devices are summarized in Table II. At 1000 cd/m2,
the EL spectral peaks and CIExy coordinates of CbPr-3
and Bim-4 host devices are at 472 nm/(0.17, 0.29) which
is found similar to mCP based device exhibiting a peak at
469 nm/(0.17, 0.28). The EL spectrum of each device does
not change significantly with the applied voltage.
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